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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations. See the links below.
Click for the questions that go with this reading
Current -Review

The term, "current," is use to describe the intensity of the charge flow. Below is an animation showing the current as it flows in a pair of wires.

Because physics describes the relationship between space, matter, time and energy, physicists describe the current as an energy flow from high energy to low energy. On a battery, or any power source, this flow is from the positive terminal to the negative terminal. This means that positive charges must be used to describe the direction of the current because positive charges are repelled from the positive terminal.

However, engineers, are focused on metal wires. What makes a metal different from other materials is the sea of available electrons. These free electrons, not positive charges, are the charges moved by the electric field. Because of this, engineers define current as the flow of negative charge. This means that when a physics student talks with an engineer about current, they must remember that the current directions are reversed. When modeling the flow of charges it does not matter if the you consider the flow of positive or negative charges. The analysis will work either way. However, physicists define current as the flow of positive charge.

Current is defined by the variable "I." (That's a capital letter "eye".) I stands for the phrase "intensity of flow." This phrase was used in some of the first publications about current. Current is defined how quickly charge flows. Charge is represent by the letter "Q" and time by "t." The current is mathematically modeled by

Note how the current is depicted as the flow of positive charges.


Ampere's Law

Ampere came up with a law that describes magnetic field due to moving charges James Maxwell fixed Ampere's equations and wrote Ampere's law down as one of Maxwell's famous 4 equations.

Maxwell's 4th Equation

Most of us would not recognize this as equation as math. ...And fortunately in this course we don't need to. Suffice it to say that Maxwell's forth equation is Ampere's Law.

We can apply this equation to charges that move in straight lines. This can be a stream of plasma or the charges moving in a wire. When we do this, Ampere's Law becomes something that is very easy to work with for most of us...

Ampere's LAw for Straight Wires

AMpere's Variable List

Magnetic fields are vectors. This means you also need to be able to find the direction of the b-field. To do this we will use the "closed right hand rule."


This video explains how to find the direction and contains a couple of examples to show you how it works.

This video can be found on YouTube at:

Remember this is in reference to the flow of positive charges. If you talk with an engineer they will tell you you are wrong and use the left hand rule. This is because engineers define current as the flow of electrons and not the flow of energy, positive particles.


Example 1 -Directions


Practice determining the b-field and current's directions with these example problems.


Solution strategy


When solving problems there are two concerns.

  1. DIRECTION of the b-field or current. This is determined by using your closed right hand. Hold your hand up in the air and work out the directions.
  2. MAGNITUDE of the numbers. For this, use the equation above.
Example 2

Click here for a slideshow showing the example's solution

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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)